In an apparatus using a secondary battery as a power source (for example, an electric motor vehicle using the secondary battery as a power source of a vehicle traveling motor), it is important performing a process such as ascertaining a degradation state of the secondary battery with accuracy, and replacing the secondary battery before a defect caused by degradation occurs. PTL 1 discloses a prediction system as a device for detecting a capacity degradation state of the secondary battery for example. In PTL 1, a method of using a root method model is described as a method of predicting the capacity degradation state.